Your AGI is not on your W-2 form. It is on last year's tax return. This is done as a security measure to keep an imposter from filing taxes in your name. If TurboTax 2018 is still on your computer along with the 2018 tax file, you can load TT 2018 and look it up. If you do not have access to that, you can ask IRS for a copy of your previous year tax return. You can do this online, which may involve a fee. Visit the website:
https://www.irs.gov/newsroom/get-prior-year-tax-information-from-the-irs